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We study a unique deposit of Nb-REE ores that occur as three orebodies overlying weathered rocks of the Tomtor ultramafic alkaline-carbonatite complex in the northern Sakha Republic (Yakutia). The formation conditions of the rich Tomtor ores remain poorly constrained. Our hypothesis is that these ores, composed of detrital and authigenic minerals, were deposited in a hydrothermal lake. The Eh-pH parameters of water are reconstructed from REE distribution in ores and in monazite, with reference to results of thermodynamic calculations. The stability fields of hydrous Ce, La, and Nd phosphates in the Pourbaix diagrams are proximal in a region within a small range of pH (4-6) and Eh (-0.25 -+0.25 V). Monazite apparently formed by dehydration of the precursor phase of hydrous REE phosphate. This inference is supported by coexistence of Fe 3+  and Fe 2+  minerals.

Layered Nb-REE ores in the Tomtor Complex (Arctic Siberia): Formation conditions